The invention relates to inductive displacement measuring systems and, more particularly, to an inspection tool which permits simultaneous visual and eddy current inspection of objects in locations for which there is difficult access; and provides an audible signal for indicating the detection and magnitude of discontinuities therein, such as, cracks, seams, or inclusions, or the degree of object displacement.
Eddy current testing utilizes the application of inductive techniques, the basic principles of which are well known. In practice, a probe coil carrying an alternating current generates a pulsating magnetic field, and when the coil is placed a nominal distance from the metal test object so that the test object is located within the pulsating magnetic field, current flow is induced within and on the surface of the test object. The induced currents, called "eddy currents" because of the circular pattern, produce a secondary a.c. magnetic field that opposes and reduces the intensity of the coil's magnetic field. Changes in the impedance of the exiting coil can then be analyzed for inspection purposes.
The classic application is in nondestructive testing. A typical procedure for such testing involves placement of the probe coil adjacent to the flat surface of a metallic test object. The influence of several physical properties of the test specimen upon the impedance characteristics of the probe coil can then be calculated for various test frequencies. With appropriate selection of an operating frequency, as determined by theory or by experiment, the depth of eddy current penetration is controlled to "look" at the surface only or into the metal itself to locate discontinuities which upset the current flow, such as cracks, or inclusions. From the impedance change it is often possible to measure, quantitatively and independently of each other, such parameters as the conductivity, dimensions, and magnetic permeability of the test object, as well as such features as the magnitude and direction of cracks, seams, inclusions, or the like.
The development of eddy current test equipment has led to numerous designs to eddy current test equipment in which test coils are connected to the electronic test instruments in a variety of measurement effects. For example, in a typical circuit as disclosed in the test "Nondestructive Testing Handbook" (edited for the Society for Nondestructive Testing, by Robert C. McMaster, Vol. II, N.Y., The Ronald Press Co., 1959, Sec. 40, pp 1-10) the sensing coil is normally a section or leg of a balanced bridge network. As a metal object moves toward the coil, or vice versa, more eddy currents are generated in the object material and losses within the bridge network increase. As the object (or coil) moves away, the losses decrease. These unbalanced conditions, produce by the impedance changes in the test coil, are sensed and converted into a signal which is directly proportional to the distance between the coil and test object.
However, irrespective of the particular form of circuit chosen, it has therefore been especially difficult to apply eddy current test methods in locations where accessibility is difficult; as, for example, small diameter bores, cylinders, and the like. In such applications, positioning of the coil relative to the test object is either impossible due to the relatively large size of the test coil and associated equipment or, at best, becomes mere guesswork. Furthermore, observing meter readings while at the same time carefully controlling the movement of the coil over the surface of the test object is very difficult, if not impossible.
Accordingly, it is an object of the invention to provide a miniature eddy current inspection tool which allows for simultaneous visual and eddy current testing of an object in relatively inaccessible locations.
It is another object to provide an eddy current inspection tool which provides an audible signal for indicating detection and magnitude of discontinuities, such as cracks, seams, or inclusions, in the test object.
It is a further object to provide a miniaturized inspection tool for the simultaneous visual and eddy current testing of objects in locations with difficult accessibility, and which can provide a first audible signal indicating detection of a discontinuity, such as a coating interface, crack, seam, or inclusion, in the test object, and a second audible signal which is frequency-dependent upon the magnitude and direction of the discontinuity or the degree of object displacement. It is still a further object to provide a miniaturized inspection tool comprising an eddy current sensor and a fiberoptic scope for visual and eddy current testing of objects in remote or difficulty accessible locations, and which is selectively operable in a first mode to provide a constant single frequency signal for signalling the detection of a discontinuity, or which can be operated in a second mode for generating a tone whose frequency is proportional to the difference between the sensor signal and a given threshold level as an indication of object displacement or the magnitude and direction of a discontinuity, such as a crack, seam, interface, or inclusion.
Additional objects, advantages, and novel features of the invention, will be set forth in part of the description which follows and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.